1. Field of the Invention
The present invention relates generally to the field of emission control equipment for boilers, heaters, kilns, or other flue gas-, or combustion gas-, generating devices (e.g., those located at power plants, processing plants, etc.) and, in particular to a new and useful method and apparatus for reducing or preventing the poisoning and/or contamination of an SCR catalyst. In another embodiment, the method and apparatus of the present invention is designed to protect the SCR catalyst.
2. Description of the Related Art
NOx refers to the cumulative emissions of nitric oxide (NO), nitrogen dioxide (NO2) and trace quantities of other nitrogen oxide species generated during combustion. Combustion of any fossil fuel generates some level of NOx due to high temperatures and the availability of oxygen and nitrogen from both the air and fuel. NOx emissions may be controlled using low NOx combustion technology and post-combustion techniques. One such post-combustion technique involves selective catalytic reduction (SCR) systems in which a catalyst facilitates a chemical reaction between NOx and a reagent (usually ammonia) to produce molecular nitrogen and water vapor.
SCR technology is used worldwide to control NOx emissions from combustion sources. This technology has been used widely in Japan for NOx control from utility boilers since the late 1970's, in Germany since the late 1980's, and in the US since the 1990's. Industrial scale SCRs have been designed to operate principally in the temperature range of 500° F. to 900° F., but most often in the range of 550° F. to 750° F. SCRs are typically designed to meet a specified NOx reduction efficiency at a maximum allowable ammonia slip. Ammonia slip is the concentration, expressed in parts per million by volume, of unreacted ammonia exiting the SCR.
For additional details concerning NOx removal technologies used in the industrial and power generation industries, the reader is referred to Steam/its generation and use, 41st Edition, Kitto and Stultz, Eds., Copyright 2005, The Babcock & Wilcox Company, Barberton, Ohio, U.S.A., particularly Chapter 34—Nitrogen Oxides Control, the text of which is hereby incorporated by reference as though fully set forth herein.
Regulations issued by the EPA promise to increase the portion of utility boilers equipped with SCRs. SCRs are generally designed for a maximum efficiency of about 90%. This limit is not set by any theoretical limits on the capability of SCRs to achieve higher levels of NOx destruction. Rather, it is a practical limit set to prevent excessive levels of ammonia slip. This problem is explained as follows.
In an SCR, ammonia reacts with NOx according to one or more of the following stoichiometric reactions (a) to (d):4NO+4NH3+O2→4N2+6H2O  (a)12NO2+12NH3→12N2+18H2O+3O2  (b)2NO2+4NH3→O2+3N2+6H2O  (c)NO+NO2+2NH3→2N2+3H2O  (d).
The above catalysis reactions occur using a suitable catalyst. Suitable catalysts are discussed in, for example, U.S. Pat. Nos. 5,540,897; 5,567,394; and 5,585,081 to Chu et al., all of which are hereby incorporated by reference as though fully set forth herein. Catalyst formulations generally fall into one of three categories: base metal, zeolite and precious metal.
Base metal catalysts use titanium oxide with small amounts of vanadium, molybdenum, tungsten or a combination of several other active chemical agents. The base metal catalysts are selective and operate in the specified temperature range. The major drawback of the base metal catalyst is its potential to oxidize SO2 to SO3; the degree of oxidation varies based on catalyst chemical formulation. The quantities of SO3 which are formed can react with the ammonia carryover to form various ammonium-sulfate salts.
Zeolite catalysts are aluminosilicate materials which function similarly to base metal catalysts. One potential advantage of zeolite catalysts is their higher operating temperature of about 970° F. (521° C.). These catalysts can also oxidize SO2 to SO3 and must be carefully matched to the flue gas conditions.
Precious metal catalysts are generally manufactured from platinum and rhodium. Precious metal catalysts also require careful consideration of flue gas constituents and operating temperatures. While effective in reducing NOx, these catalysts can also act as oxidizing catalysts, converting CO to CO2 under proper temperature conditions. However, SO2 oxidation to SO3 and high material costs often make precious metal catalysts less attractive.
As is known to those of skill in the art, various SCR catalysts undergo poisoning when they become contaminated by various compounds including, but not limited to, certain phosphorus compounds such as phosphorous oxide (PO) or phosphorous pentoxide (P2O5). Additionally, certain compounds that contain potassium (K), sodium (Na) and phosphorous (P) that are found in, or generated by, various coal-based fuels are known to cause rapid deactivation of SCR catalyst in full-scale units and also in slip-stream units. In these fuels, potassium and sodium are mainly in the form of organically bonded inorganics, or water soluble salts, (see, e.g., Steenari et al.; Energy and Fuels; Vol. 18 (2004) 6, pp. 1870 to 1876). This form of association in the fuel makes it very easy for potassium and sodium to vaporize during combustion. Phosphorus can also be contained in the fuel where it is organically bonded (as in the case in biomass) or inorganically bonded (as is the case in Powder River Basin (PRB) coal). Phosphorus is released in the gas phase due to carbothermic reduction reaction happening during char combustion as follows:P2O5(solid phase compounds)+3C(s)→2PO(g)+3CO(g)(see, e.g., Hino, et. al.; ISIJ International, Vol. 48 (2008)7, pp. 912 to 917). Of particular concern with biomass, phosphorus is released in the gas phase as a result of the combustion process itself irrespective of whether the combustion is staged or un-staged since phosphorus is organically associated with/in the fuel.
More particularly, as the SCR catalysts are exposed to the dust laden flue gas there are numerous mechanisms including blinding, masking and poisoning that deactivates the catalyst and causes a decrease in the catalyst's performance over time. The most common catalyst poison encountered when burning eastern domestic coal (i.e., coal mined in the eastern United States) is arsenic. The most common catalyst poison encountered when burning western domestic coal (i.e., coal mined in the western United States) is phosphorus and calcium sulfate is the most common masking mechanism. The most common catalyst poisons encountered when burning biomass are typically potassium and sodium, or potassium- and sodium-containing compounds. One method of recycling the used catalyst is the process called regeneration washing or rejuvenation. The initial steps of the regeneration process involve the removal of these toxic chemicals by processing the catalysts through various chemical baths in which the poisons are soluble. While this treatment process does an excellent job of removing the desired poisons it produces wastewater with very high arsenic concentrations.
Furthermore, as is known to those of skill in the art, selective catalyst reduction (SCR) technology is used worldwide to control NOx emissions from combustion sources at high temperatures (550° F. to 750° F.). High temperature SCR technology has been used in Japan for NOx control from utility boilers since the late 1970s, in Germany since the late 1980's, in US since the 1990's and in China since 2000. The function of the SCR system is to react NOx with ammonia (NH3) and oxygen to form molecular nitrogen and water. Due to anticipated requirements for lower NOx emission limits there is a growing need to control NOx emissions from lignite fired coal power plants in the US and Canada. Some lignite fired units are already in the process of retrofitting SCR to control NOx. Other units will have to follow suit in the near future. There is also an increasing trend to co-combust coal and biomass on existing units with or without SCR. Some older units are completely switching from pulverized coal firing to pulverized biomass combustion. These units even with biomass alone or with coal and biomass co-combustion will have to comply with strict NOx emissions. The most effective method of complying with low NOx emission requirements is by SCR technology. The main issue with SCR performance on these units is the deactivation of the catalyst. Both lignite and biomass fuels have potassium, sodium and phosphorous and/or various potassium, sodium and phosphorous compounds which are known catalyst poisons. These poisons attack the catalyst resulting in deactivation of the catalyst over a period of time, thereby shortening the catalyst's active life cycle. As a result of the deactivation, the catalyst cannot function to carry out NOx reduction as effectively for a longer period of time. Given this, the deactivation reduces the effective life cycle of a catalyst and as a result more frequent catalyst changes are needed for NOx compliance. Although, there are some catalyst vendors that claim resistance to arsenic poisoning of their catalysts via the use of molybdenum in the catalyst formulation, to date no catalyst have been brought to market that resist poisoning by various potassium, sodium and phosphorous compounds, their elemental species, or their ionic species.
Additionally, beyond controlling NOx emissions, other emission controls must be considered and/or met in order to comply with various state, EPA and/or Clean Air Act regulations. Some other emission controls which need to be considered for boilers, heaters, kilns, or other flue gas-, or combustion gas-, generating devices (e.g., those located at power plants, processing plants, etc.) include, but are not limited to, mercury, SOx, and certain particulates.
Given the above, a need exists for a method that provides for an economical and environmentally suitable method and/or system to remove the gaseous potassium, sodium and phosphorous compounds, their elemental species, or their ionic species from a combustion process prior to any phosphorus compounds poisoning a catalyst in an SCR.